Chapter 6 - Chapter 6 Regulation of Effective Circulating...

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Chapter 6 Regulation of Effective Circulating Volume (ECV) and NaCl Balance NaCl is the major solute of ECF, which consists of plasma and ISF. The amount of NaCl in ECF determines ECF osmolality This, in turn determines the blood volume and blood pressure. Renal mechanisms involved in reabsorption of Na + (i.e., returning Na + to ECF after filtration), are critically important for the maintenance of normal ECF volume, normal blood volume, and normal blood pressure. The kidneys are responsible for maintaining a normal body Na + content. On a daily basis, kidneys must ensure that Na + excretion exactly equals Na + intake, a matching process called Na + balance . To remain in Na + balance, a person who ingests 150 mEq of Na + daily must excrete exactly 150 mEq of Na + daily. Changes in Na + balance do not normally alter ECF osmolality. Why? - ADH and thirst systems maintain body fluid osmolality within a narrow range. Kidneys are major routes for excretion of NaCl from the body. - Play a major role in regulating the volume of ECF. - Normal conditions : Kidneys keep volume of ECF constant by adjusting excretion of NaCl to match amount ingested. … Ingestion > excretion ECF volume … Ingestion < excretion ECF volume - Body monitors volume of ECF and sends signals to kidneys … Make adjustments in NaCl excretion The Concept of Effective Circulating Volume (ECV) ECV is not measurable Refers to the portion of ECF volume contained within the vascular system, and is “effectively” perfusing the tissues. Dependent on: - Volume (fullness) – within vascular system - Pressure – within vascular system - Also related to cardiac output Normal individuals: - ECV varies directly with the: … ECF volume (arterial and venous) … Arterial blood pressure … Cardiac output (CO) - A decrease in ECF & vascular volume, arterial pressure, or CO - Opposite is true for an increase in ECF … ECF volume, vascular volume, arterial BP and CO depend on: … Effective Circulating Volume (ECV) … Related to Na + balance
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